System and method for using impedance to determine proximity and orientation of segmented electrodes

ABSTRACT

A method for implanting a neurostimulation lead within a patient includes measuring impedances of electrodes on the lead in order to correctly position the lead relative to a target tissue region. The electrodes are circumferentially segmented electrodes that are spaced from each other about the longitudinal axis of the lead. When the difference between the impedances of the electrodes exceeds a threshold value, the lead is in the correct position. In accordance with another embodiment, impedance measurements are used to select which one of the electrodes is closest to the target tissue region. By determining which electrode has the highest impedance and which electrode has the lowest impedance, the type of tissue adjacent to each electrode can be determined based on the conductivity properties of the tissue. The target tissue region may be a spinal cord, a posterior longitudinal ligament, white matter, or gray matter.

RELATED APPLICATION DATA

The present application is a continuation of U.S. application Ser. No.15/993,897, filed May 31, 2018, which is a continuation of U.S.application Ser. No. 15/363,557, filed Nov. 29, 2016, now issued as U.S.Pat. No. 10,016,594, which is a continuation of U.S. application Ser.No. 13/533,532, filed Jun. 26, 2012, now issued as U.S. Pat. No.9,511,229, which claims the benefit under 35 U.S.C. § 119 to U.S.provisional patent application Ser. No. 61/502,192, filed Jun. 28, 2011.The foregoing applications are hereby incorporated by reference into thepresent application in their entirety.

FIELD OF THE INVENTION

The present inventions relate to tissue stimulation systems, and moreparticularly, to systems and methods for determining proximity andorientation of segmented stimulation electrodes relative to targettissue.

BACKGROUND OF THE INVENTION

Implantable neurostimulation systems have proven therapeutic in a widevariety of diseases and disorders. Pacemakers and Implantable CardiacDefibrillators (ICDs) have proven highly effective in the treatment of anumber of cardiac conditions (e.g., arrhythmias). Spinal CordStimulation (SCS) systems have long been accepted as a therapeuticmodality for the treatment of chronic pain syndromes, peripheralvascular disease, and angina pectoralis, and the application of tissuestimulation has begun to expand to additional applications such as heartfailure and incontinence. Deep Brain Stimulation (DBS) has also beenapplied therapeutically for well over a decade for the treatment ofrefractory chronic pain syndromes, and DBS has also recently beenapplied in additional areas such as movement disorders and epilepsy.Further, Functional Electrical Stimulation (FES) systems such as theFreehand system by NeuroControl (Cleveland, Ohio) have been applied torestore some functionality to paralyzed extremities in spinal cordinjury patients. Furthermore, in recent investigations Peripheral NerveStimulation (PNS) systems have demonstrated efficacy in the treatment ofchronic pain syndromes and incontinence, and a number of additionalapplications are currently under investigation. Occipital NerveStimulation (ONS), in which leads are implanted in the tissue over theoccipital nerves, has shown promise as a treatment for variousheadaches, including migraine headaches, cluster headaches, andcervicogenic headaches.

These implantable neurostimulation systems typically include one or moreelectrode carrying neurostimulation leads, which are implanted at thedesired stimulation site, and a neurostimulator (e.g., an implantablepulse generator (IPG)) implanted remotely from the stimulation site, butcoupled either directly to the neurostimulation lead(s) or indirectly tothe neurostimulation lead(s) via a lead extension. Thus, electricalpulses can be delivered from the neurostimulator to the neurostimulationleads to stimulate the tissue and provide the desired efficacioustherapy to the patient. The neurostimulation system may further comprisea handheld patient programmer in the form of a remote control (RC) toremotely instruct the neurostimulator to generate electrical stimulationpulses in accordance with selected stimulation parameters. A typicalstimulation parameter set may include the electrodes that are acting asanodes or cathodes, as well as the amplitude, duration, and rate of thestimulation pulses. The RC may, itself, be programmed by a clinician,for example, by using a clinician's programmer (CP), which typicallyincludes a general purpose computer, such as a laptop, with aprogramming software package installed thereon. Typically, the RC canonly control the neurostimulator in a limited manner (e.g., by onlyselecting a program or adjusting the pulse amplitude or pulse width),whereas the CP can be used to control all of the stimulation parameters,including which electrodes are cathodes or anodes.

In the context of an SCS procedure, one or more neurostimulation leadsare introduced through the patient's back into the epidural space, suchthat the electrodes carried by the leads are arranged in a desiredpattern and spacing to create an electrode array. One type ofcommercially available neurostimulation lead is a percutaneous lead,which can be introduced proximal to the affected spinal tissue through aTouhy-like needle, which passes through the skin, between the desiredvertebrae, and is positioned epidurally. For unilateral pain, apercutaneous lead is placed on the corresponding lateral side of thespinal cord. For bilateral pain, a percutaneous lead is placed down themidline of the spinal cord, or two or more percutaneous leads are placeddown the respective sides of the midline of the spinal cord, and if athird lead is used, down the midline of the spinal cord. After properplacement of the neurostimulation leads at the target area of the spinalcord, the leads are anchored in place at an exit site to preventmovement of the neurostimulation leads. To facilitate the location ofthe neurostimulator away from the exit point of the neurostimulationleads, lead extensions are sometimes used.

The neurostimulation leads, or the lead extensions, are then connectedto the IPG, which can then be operated to generate electrical pulsesthat are delivered, through the electrodes, to the targeted tissue. Inthe particular case of traditional stimulation treatment of chronicpain, this tissue is typically the dorsal column and dorsal root fiberswithin the spinal cord. Such stimulation creates the sensation known asparesthesia, which can be characterized as an alternative sensation thatreduces the pain signals sensed by the patient. Intra-operatively (i.e.,during the surgical procedure), the neurostimulator may be operated totest the effect of stimulation and adjust the parameters of thestimulation for optimal pain relief. The patient may provide verbalfeedback regarding the presence of paresthesia over the pain area, andbased on this feedback, the lead positions may be adjusted andre-anchored if necessary. A computer program, such as Bionic Navigator®,available from Boston Scientific Neuromodulation Corporation, can beincorporated in a clinician's programmer (CP) (briefly discussed above)to facilitate selection of the stimulation parameters. Any incisions arethen closed to fully implant the system. Post-operatively (i.e., afterthe surgical procedure has been completed), a clinician can adjust thestimulation parameters using the computerized programming system tore-optimize the therapy.

The efficacy of SCS is related to the ability to stimulate the spinalcord tissue corresponding to evoked paresthesia in the region of thebody where the patient experiences pain. Thus, the working clinicalparadigm is that achievement of an effective result from SCS depends onthe neurostimulation lead or leads being placed in a location (bothlongitudinal and lateral) relative to the spinal tissue such that theelectrical stimulation will induce paresthesia located in approximatelythe same place in the patient's body as the pain (i.e., the target oftreatment). If a lead is not correctly positioned, it is possible thatthe patient will receive little or no benefit from an implanted SCSsystem. Thus, correct lead placement can mean the difference betweeneffective and ineffective pain therapy, and as such, precise positioningof the leads proximal to the targets of stimulation is critical to thesuccess of the therapy.

However, percutaneous leads are typically implanted near target tissueusing a “blind” needle approach. Imaging tools may be used to assist incorrectly placing the leads near the target tissue, but imaging toolsmay be imprecise in terms of guiding placement. For example, becausesoft tissue is not shown in an x-ray, only the position of the leadrelative to the vertebral bones of the spinal column is shown in anx-ray. Therefore, it is almost impossible to tell how close the lead isto the dura of the spinal cord when looking at an x-ray. Additionally,if the electrodes on the leads are segmented circumferential electrodes(e.g., as described in U.S. Provisional Patent Application Ser. No.61/427,441, which is expressly incorporated herein by reference), theorientation of the electrodes intended for applying stimulation to thetargets may not be known or controlled.

There, thus, remains a need for a system and method for preciselydetermining the location and orientation of segmented circumferentialelectrodes relative to target tissue.

SUMMARY OF THE INVENTION

Each of the methods described below uses a neurostimulation lead havinga longitudinal axis and an electrode ring segmented into at least twoelectrodes circumferentially spaced from each other about thelongitudinal axis. The at least two electrodes may be diametricallydisposed from each other. Each of the systems described below are foruse with the neurostimulation lead, as described above, positionedrelative to a target tissue region.

In accordance with a first aspect of the present inventions, a methodfor of performing a medical procedure on a patient is provided. Themethod includes positioning the neurostimulation lead within the patientrelative to a target tissue region. The target tissue region may be aspinal cord, a posterior longitudinal ligament, white matter, or graymatter. For example, positioning the neurostimulation lead within thepatient may include positioning the neurostimulation lead in an epiduralspace such that the target tissue region is the spinal cord. In anotherexample, positioning the neurostimulation lead within the patient mayinclude positioning the neurostimulation lead in a ventral region of theepidural space such that the target tissue region is the posteriorlongitudinal ligament. In yet another example, positioning theneurostimulation lead within the patient may include positioning theneurostimulation lead between the spinal cord and a dura, such that thetarget tissue region is the spinal cord. In still another example,positioning the neurostimulation lead within the patient may includepositioning the neurostimulation lead in a brain, such that the targettissue region is gray matter or white matter.

The method further includes measuring an impedance of each of the twoelectrodes; determining the difference between the impedances of the twoelectrodes; comparing the impedance difference to a threshold value; andre-positioning the neurostimulation lead relative to the target tissueregion based on the comparison. Re-positioning the neurostimulation leadmay include linearly displacing the neurostimulation lead closer to thetarget tissue region. In addition or alternatively, re-positioning theneurostimulation lead may include rotating the neurostimulation leadabout the longitudinal axis to locate one of the electrodes closer tothe target tissue region. The method may further include determiningthat the impedance difference is less than the threshold value. Themethod may further include affixing the neurostimulation lead afterre-positioning the neurostimulation lead.

In accordance with a second aspect of the present inventions, a methodfor providing therapy to a patient is provided. The method includesmeasuring an impedance of each of the electrodes, and, based on theimpedance measurement, selecting the electrode closest to the targettissue region. For example, the electrode having the lowest impedancemay be selected to be the electrode closest to the target tissue region.In another example, the electrode having the highest impedance may beselected to be the electrode closest to the target tissue region. Themethod further includes conveying electrical stimulation energy from theselected electrode.

In one embodiment of the second aspect of the present inventions, theneurostimulation lead may be implanted within a ventral region of anepidural space of the patient, the target tissue region may be aposterior longitudinal ligament, and the electrode having the highestimpedance may be selected to be the electrode closest to the posteriorlongitudinal ligament. In another embodiment of the second aspect of thepresent inventions, the neurostimulation lead may be implanted within anepidural space of the patient, the target tissue region may be a spinalcord, and the electrode having the lowest impedance may be selected tobe the electrode closest to the spinal cord. In yet another embodimentof the second aspect of the present inventions, the neurostimulationlead may be implanted between a dura and a spinal cord, the targettissue region may be the spinal cord, and the electrode having thehighest impedance may be selected to be the electrode closest to thespinal cord. In still another embodiment of the second aspect of thepresent inventions, the neurostimulation lead may be implanted within abrain of the patient with one of the electrodes adjacent to white matterand the other electrode adjacent to gray matter, and the target tissueregion may be the white matter or the gray matter.

In accordance with a third aspect of the present inventions, a systemfor providing therapy to a patient is provided. The system includes animpedance monitor configured for being coupled to the electrodes and formeasuring an impedance of each of the electrodes. The impedance monitormay be configured for continuously measuring the impedance of each ofthe electrodes. The system further includes a processor configured fordetermining a difference between the impedances of the electrodes, forcomparing the difference to a threshold value, and for providing asuggestion to a user to re-position the neurostimulation lead within thepatient based on the comparison. The suggestion may be to displace theneurostimulation lead closer to the target tissue region. In addition,or alternatively, the suggestion may be to rotate the neurostimulationlead about the longitudinal axis to locate one of the electrodes closerto the target tissue region. The target tissue region may be a spinalcord, a posterior longitudinal ligament, white matter, or gray matter.The processor may be configured to provide the suggestion to re-positionthe neurostimulation lead to the user only if the impedance differenceis less than the threshold value.

In accordance with a fourth aspect of the present inventions, a systemfor providing therapy to a patient is provided. The system includes animpedance monitor configured for being coupled to the electrodes and formeasuring an impedance of each of the electrodes, and a processorconfigured for, based on the impedance measurement, selecting theelectrode closest to the target tissue region. The processor may beconfigured for selecting the electrode having the lowest impedance to bethe electrode closest to the target tissue region. Alternatively, theprocessor may be configured for selecting the electrode having thehighest impedance to be the electrode closest to the target tissueregion. The system also includes a controller configured for instructinga neurostimulator to convey electrical stimulation energy from theselected electrode.

In one embodiment of the fourth aspect of the present inventions, theneurostimulation lead may be implanted within a ventral portion of anepidural space of the patient, the target tissue region may be aposterior longitudinal ligament, and the processor may be configured forselecting the electrode having the highest impedance to be the electrodeclosest to the posterior longitudinal ligament. In another embodiment ofthe fourth aspect of the present inventions, the neurostimulation leadmay be implanted within an epidural space of the patient, the targettissue region may be a spinal cord of the patient, and the processor maybe configured for selecting the electrode having the lowest impedance tobe the electrode closest to the spinal cord. In yet another embodimentof the fourth aspect of the present inventions, the neurostimulationlead may be implanted within a brain of the patient with one of theelectrodes adjacent to white matter and the other electrode adjacent togray matter, and the target tissue region may be one of the white matterand the gray matter. In still another embodiment of the fourth aspect ofthe present inventions, the neurostimulation lead may be implantedbetween a spinal cord and a dura, the target tissue region may be thespinal cord, and the processor may be configured for selecting theelectrode having the highest impedance to be the electrode closest tothe spinal cord.

Other and further aspects and features of the invention will be evidentfrom reading the following detailed description of the preferredembodiments, which are intended to illustrate, not limit, the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings illustrate the design and utility of preferred embodimentsof the present invention, in which similar elements are referred to bycommon reference numerals. In order to better appreciate how theabove-recited and other advantages and objects of the present inventionsare obtained, a more particular description of the present inventionsbriefly described above will be rendered by reference to specificembodiments thereof, which are illustrated in the accompanying drawings.Understanding that these drawings depict only typical embodiments of theinvention and are not therefore to be considered limiting of its scope,the invention will be described and explained with additionalspecificity and detail through the use of the accompanying drawings inwhich:

FIG. 1 is a plan view of an embodiment of a neurostimulation systemarranged in accordance with the present inventions;

FIG. 2 is a profile view of an implantable pulse generator (IPG) andneurostimulation leads used in the neurostimulation system of FIG. 1;

FIG. 3 is a cross-sectional view of a neurostimulation lead used in theneurostimulation system of FIG. 1;

FIG. 4 is a block diagram of the internal components of the IPG of FIG.1;

FIG. 5 is a plan view of the neurostimulation system of FIG. 1 in usewithin a patient in accordance with one embodiment of the presentinventions;

FIG. 6 is a plan view of the neurostimulation system of FIG. 1 in usewithin a patient in accordance with another embodiment of the presentinventions;

FIGS. 7A-7E are cross-sectional views of a spinal canal with aneurostimulation lead implanted therein in various positions;

FIG. 7F is a cross-sectional view of a neurostimulation lead implantedwithin brain tissue;

FIG. 8 is a flow chart illustrating a method of implanting aneurostimulation lead in accordance with one embodiment of the presentinventions; and

FIG. 9 is a flow chart illustrating a method of selecting one of theelectrodes on the neurostimulation lead for conveying stimulation to thetarget tissue region.

DETAILED DESCRIPTION OF THE EMBODIMENTS

At the outset, it is noted that the present invention may be used withan implantable pulse generator (IPG), radio frequency (RF) transmitter,or similar neurostimulator, that may be used as a component of numerousdifferent types of stimulation systems. The description that followsrelates to a deep brain stimulation (DBS) system and a spinal cordstimulation (SCS) system. However, it is to be understood that the whilethe invention lends itself well to applications in DBS and SCS, theinvention, in its broadest aspects, may not be so limited. Rather, theinvention may be used with any type of implantable electrical circuitryused to stimulate tissue. For example, the present invention may be usedas part of a pacemaker, a defibrillator, a cochlear stimulator, aretinal stimulator, a stimulator configured to produce coordinated limbmovement, a cortical stimulator, a peripheral nerve stimulator, amicrostimulator, or in any other neural stimulator configured to treaturinary incontinence, sleep apnea, shoulder sublaxation, headache, etc.

Turning first to FIG. 1, an exemplary neurostimulation system 10generally includes at least one implantable neurostimulation lead 12 (inthis case, two), a neurostimulator in the form of an implantable pulsegenerator (IPG) 14, a remote controller RC 16, a clinician's programmer(CP) 18, an External Trial Stimulator (ETS) 20, and an external charger22.

The IPG 14 is physically connected via one or more percutaneous leadextensions 24 to the neurostimulation leads 12, which carry a pluralityof electrodes 26 arranged in an array. In the illustrated embodiment,the neurostimulation leads 12 are percutaneous leads, and to this end,the electrodes 26 are arranged in-line along the neurostimulation leads12. As will be described in further detail below, the IPG 14 includespulse generation circuitry that delivers electrical stimulation energyin the form of a pulsed electrical waveform (i.e., a temporal series ofelectrical pulses) to the electrode array 26 in accordance with a set ofstimulation parameters.

The ETS 20 may also be physically connected via the percutaneous leadextensions 28 and external cable 30 to the neurostimulation leads 12.The ETS 20, which has pulse generation circuitry similar to that of theIPG 14, also delivers electrical stimulation energy in the form of apulsed electrical waveform to the electrode array 26 in accordance witha set of stimulation parameters. The major difference between the ETS 20and the IPG 14 is that the ETS 20 is a non-implantable device that isused, on a trial basis after the neurostimulation leads 12 have beenimplanted and prior to implantation of the IPG 14, to test theresponsiveness of the stimulation that is to be provided. Thus, anyfunctions described herein with respect to the IPG 14 can likewise beperformed with respect to the ETS 20.

The RC 16 may be used to telemetrically control the ETS 20 via abi-directional RF communications link 32. Once the IPG 14 andneurostimulation leads 12 are implanted, the RC 16 may be used totelemetrically control the IPG 14 via a bi-directional RF communicationslink 34. Such control allows the IPG 14 to be turned on or off and to beprogrammed with different stimulation parameter sets. The IPG 14 mayalso be operated to modify the programmed stimulation parameters toactively control the characteristics of the electrical stimulationenergy output by the IPG 14.

The CP 18 provides clinician detailed stimulation parameters forprogramming the IPG 14 and ETS 20 in the operating room and in follow-upsessions. The CP 18 may perform this function by indirectlycommunicating with the IPG 14 or ETS 20, through the RC 16, via an IRcommunications link 36. Alternatively, the CP 18 may directlycommunicate with the IPG 14 or ETS 20 via an RF communications link (notshown). The clinician detailed stimulation parameters provided by the CP18 are also used to program the RC 16, so that the stimulationparameters can be subsequently modified by operation of the RC 16 in astand-alone mode (i.e., without the assistance of the CP 18).

The external charger 22 is a portable device used to transcutaneouslycharge the IPG 14 via an inductive link 38. Once the IPG 14 has beenprogrammed, and its power source has been charged by the externalcharger 22 or otherwise replenished, the IPG 14 may function asprogrammed without the RC 16 or CP 18 being present.

For purposes of brevity, the details of the RC 16, CP 18, ETS 20, andexternal charger 22 will not be described herein. Details of exemplaryembodiments of these devices are disclosed in U.S. Pat. No. 6,895,280,which is expressly incorporated herein by reference.

Referring to FIG. 2, the IPG 14 comprises an outer case 40 for housingthe electronic and other components, and a connector 42 to which theproximal end of the neurostimulation lead 12 mates in a manner thatelectrically couples the electrodes 26 to the internal electronicswithin the outer case 40. The outer case 40 is composed of anelectrically conductive, biocompatible material, such as titanium, andforms a hermetically sealed compartment wherein the internal electronicsare protected from the body tissue and fluids. In some cases, the outercase 40 may serve as an electrode.

Each of the neurostimulation leads 12 comprises an elongated cylindricallead body 43, and the electrodes 26 take the form of segmentedelectrodes that are circumferentially and axially disposed about thelead body 43. By way of non-limiting example, and with further referenceto FIG. 3, each neurostimulation lead 12 may carry eight electrodes,arranged as four rings of electrodes (the first ring consisting ofelectrodes E1 and E2; the second ring consisting of electrodes E3 andE4; the third ring consisting of electrodes E5 and E6; and the fourthring consisting of electrodes E7 and E8) or two axial columns ofelectrodes (the first column consisting of electrodes E1, E3, E5, andE7; the second column consisting of electrodes E2, E4, E6, and E8). Thetwo electrodes 26 a, 26 b in each electrode ring are diametricallydisposed from each other. The actual number and shape of leads andelectrodes will, of course, vary according to the intended application.For example, each ring of electrodes may include more than twoelectrodes. Further details describing the construction and method ofmanufacturing percutaneous stimulation leads are disclosed in U.S.patent application Ser. No. 11/689,918, entitled “Lead Assembly andMethod of Making Same,” and U.S. patent application Ser. No. 11/565,547,entitled “Cylindrical Multi-Contact Electrode Lead for NeuralStimulation and Method of Making Same,” the disclosures of which areexpressly incorporated herein by reference.

The IPG 14 includes a battery and pulse generation circuitry thatdelivers the electrical stimulation energy in the form of a pulsedelectrical waveform to the electrode array 26 in accordance with a setof stimulation parameters programmed into the IPG 14. Such stimulationparameters may comprise electrode combinations, which define theelectrodes that are activated as anodes (positive), cathodes (negative),and turned off (zero), percentage of stimulation energy assigned to eachelectrode (fractionalized electrode configurations), and electricalpulse parameters, which define the pulse amplitude (measured inmilliamps or volts depending on whether the IPG 14 supplies constantcurrent or constant voltage to the electrode array 26), pulse duration(measured in microseconds), pulse rate (measured in pulses per second),and burst rate (measured as the stimulation on duration X andstimulation off duration Y).

Electrical stimulation will occur between two (or more) activatedelectrodes, one of which may be the IPG case 40. Simulation energy maybe transmitted to the tissue in a monopolar or multipolar (e.g.,bipolar, tripolar, etc.) fashion. Monopolar stimulation occurs when aselected one of the lead electrodes 26 is activated along with the case40 of the IPG 14, so that stimulation energy is transmitted between theselected electrode 26 and case 40. Bipolar stimulation occurs when twoof the lead electrodes 26 are activated as anode and cathode, so thatstimulation energy is transmitted between the selected electrodes 26.Tripolar stimulation occurs when three of the lead electrodes 26 areactivated, two as anodes and the remaining one as a cathode, or two ascathodes and the remaining one as an anode.

Turning next to FIG. 4, the main internal components of the IPG 14 willnow be described. The IPG 14 includes stimulation output circuitry 60configured for generating the electrical stimulation energy undercontrol of control logic 62 over data bus 64. Control of the pulse rateand pulse width of the electrical waveform is facilitated by timer logiccircuitry 66, which may have a suitable resolution, e.g., 10 μs. Thestimulation energy generated by the stimulation output circuitry 60 isoutput via capacitors C1-C16 to electrical terminals 68 corresponding tothe electrodes 26.

The analog output circuitry 60 may either comprise independentlycontrolled current sources for providing stimulation pulses of aspecified and known amperage to or from the electrical terminals 68, orindependently controlled voltage sources for providing stimulationpulses of a specified and known voltage at the electrical terminals 68or to multiplexed current or voltage sources that are then connected tothe electrical terminals 68. The operation of this analog outputcircuitry, including alternative embodiments of suitable outputcircuitry for performing the same function of generating stimulationpulses of a prescribed amplitude and width, is described more fully inU.S. Pat. Nos. 6,516,227 and 6,993,384, which are expressly incorporatedherein by reference.

The IPG 14 further comprises monitoring circuitry 70 for monitoring thestatus of various nodes or other points 72 throughout the IPG 14, e.g.,battery voltage, temperature, and the like. Significantly, themonitoring circuitry 70 is configured for taking impedance measurements,so that, as will be described in further detail below, each of the leads12 may be positioned correctly relative to a target tissue region and/orthe electrode or electrodes closest to the target tissue region can beactivated. The monitoring circuitry 70 may also measure the impedance ateach electrode 26 in order to determine the coupling efficiency betweenthe respective electrode 26 and the tissue and/or to facilitate faultdetection with respect to the connection between the electrodes 26 andthe analog output circuitry 60 of the IPG 14.

Impedance can be measured using any one of a variety means. For example,the impedance can be measured on a sampled basis during a portion of thetime while the electrical stimulus pulse is being applied to the tissue,or immediately subsequent to stimulation, as described in U.S. patentapplication Ser. No. 10/364,436, which has previously been incorporatedherein by reference. Alternatively, the impedance can be measuredindependently of the electrical stimulation pulses, such as described inU.S. Pat. Nos. 6,516,227 and 6,993,384, which were previouslyincorporated herein by reference.

To facilitate determination of the location of each neurostimulationlead 12, electrical signals can be transmitted between electrodescarried by one of the neurostimulation leads 12 and one or more otherelectrodes (e.g., electrodes on the same neurostimulation lead 12,electrodes on the other neurostimulation lead 12, the case 40 of the IPG14, or an electrode affixed to the tissue), and then electricalimpedance can be measured in response to the transmission of theelectrical signals.

The IPG 14 further comprises processing circuitry in the form of amicrocontroller 74 that controls the control logic 62 over data bus 76,and obtains status data from the monitoring circuitry 70 via data bus78. The microcontroller 74 additionally controls the timer logic 66. TheIPG 14 further comprises memory 80 and an oscillator and clock circuit82 coupled to the microcontroller 74. The microcontroller 74, incombination with the memory 80 and oscillator and clock circuit 82, thuscomprise a microprocessor system 81 that carries out a program functionin accordance with a suitable program stored in the memory 80.Alternatively, for some applications, the function provided by themicroprocessor system may be carried out by a suitable state machine.

Thus, the microcontroller 74 generates the necessary control and statussignals, which allow the microcontroller 74 to control the operation ofthe IPG 14 in accordance with a selected operating program andparameters. In controlling the operation of the IPG 14, themicrocontroller 74 is able to individually generate electrical pulses atthe electrodes 26 using the analog output circuitry 60, in combinationwith the control logic 62 and timer logic 66, thereby allowing eachelectrode 26 to be paired or grouped with other electrodes 26, includingthe monopolar case electrode, and to control the polarity, amplitude,rate, and pulse width through which the current stimulus pulses areprovided.

Significantly, the microcontroller 74 is able to obtain impedance datafrom the monitoring circuitry 70. As discussed in more detail below, themicroprocessor system 81 is configured for analyzing the impedance datain order to determine whether the neurostimulation lead 12 and theelectrodes 26 are positioned correctly during implantation. Themicroprocessor system 81 may also be configured for providing asuggestion to a user to re-position the neurostimulation lead 12. Inaddition or alternatively, the microprocessor system 81 is configuredfor analyzing the impedance data in order to determine which of theelectrodes 26 is closest to the target tissue region and for instructingthe IPG 14 to convey electrical stimulation energy from that electrode.Alternatively or additionally, the CP 18 and/or the RC 16 may includeprocessors and controllers for performing the tasks of obtaining theimpedance data, analyzing the impedance data, determining whether theneurostimulation lead 12 and electrodes 26 are positioned correctly,providing a suggestion to a user to re-position the neurostimulationlead 12, determining which of the electrodes 26 is closest to the targettissue region, and instructing the IPG 14 to convey electricalstimulation energy from the electrode that is closest to the targettissue region.

The IPG 14 further comprises an alternating current (AC) receiving coil84 for receiving programming data (e.g., the operating program and/orstimulation parameters) from the RC 16 in an appropriate modulatedcarrier signal, and charging and forward telemetry circuitry 86 fordemodulating the carrier signal it receives through the AC receivingcoil 84 to recover the programming data, which programming data is thenstored within the memory 80, or within other memory elements (not shown)distributed throughout the IPG 14.

The IPG 14 further comprises back telemetry circuitry 88 and analternating current (AC) transmission coil 90 for sending informationaldata (e.g., the impedance data) to the RC 16. The back telemetryfeatures of the IPG 14 also allow its status to be checked. For example,any changes made to the stimulation parameters are confirmed throughback telemetry, thereby assuring that such changes have been correctlyreceived and implemented within the IPG 14. Moreover, upon interrogationby the RC 16, all programmable settings stored within the IPG 14 may beuploaded to the RC 16.

The IPG 14 further comprises a replenishable power source 92, which may,e.g., comprise a rechargeable battery, such as a lithium-ion orlithium-ion polymer battery. The rechargeable battery 92 is rechargedusing rectified AC power (or DC power converted from AC power throughother means, e.g., efficient AC-to-DC converter circuits) received bythe AC receiving coil 84. To recharge the battery 92, the externalcharger 22, which generates the AC magnetic field, is placed against, orotherwise adjacent, to the patient's skin over the implanted IPG 14. TheAC magnetic field emitted by the external charger 22 induces AC currentsin the AC receiving coil 84. The charging and forward telemetrycircuitry 86 rectifies the AC current to produce DC current, which isused to charge the battery 92.

The IPG 14 further comprises power circuits 94 to which the rechargeablebattery 92 provides an unregulated voltage. The power circuits 94, inturn, generate the various voltages 96, some of which are regulated andsome of which are not, as needed by the various circuits located withinthe IPG 14 for providing the operating power to the IPG 14. The powercircuits 94 also include protection circuitry that protects therechargeable battery 92 from overcharging. Also, safeguarding featuresare incorporated that assure that the battery 92 is always operated in asafe mode upon approaching a charge depletion. Potentially endangeringfailure modes are avoided and prevented through appropriate logiccontrol that is hard-wired into the IPG 14, or otherwise set in the IPG14 in such a way that the patient cannot override them.

The neurostimulation system 10 described above may be used for DBS orSCS. With DBS, as shown in FIG. 5, the two percutaneous neurostimulationleads 12 may be introduced through a burr hole 46 (or alternatively, tworespective burr holes) formed in the cranium 48 of a patient 44, andintroduced into the parenchyma of the brain 49 of the patient 44 in aconventional manner, such that the electrodes 26 are adjacent a targettissue region. As discussed in more detail below, the target tissueregion may be gray matter or white matter. Due to the lack of space nearthe location where the neurostimulation leads 12 exit the burr hole 46,the IPG 14 is generally implanted in a surgically-made pocket either inthe chest, or in the abdomen. The IPG 14 may, of course, also beimplanted in other locations of the patient's body. The leadextension(s) 24 facilitates locating the IPG 14 away from the exit pointof the neurostimulation leads 12.

With SCS, as shown in FIG. 6, the neurostimulation leads 12 areimplanted within the spinal column 52 of a patient 50, such that theelectrodes 26 are adjacent to target tissue. As discussed in more detailbelow, the target tissue in SCS may be, for example, the spinal cord orthe posterior longitudinal ligament. Due to the lack of space near thelocation where the neurostimulation leads 12 exit the spinal column 52,the IPG 14 is generally implanted in a surgically-made pocket either inthe abdomen or above the buttocks. The IPG 14 may, of course, also beimplanted in other locations of the patient's body. The lead extension24 facilitates locating the IPG 14 away from the exit point of theneurostimulation leads 12. As there shown, the CP 18 communicates withthe IPG 14 via the RC 16.

As previously discussed, correctly placing the neurostimulation lead 12near the target tissue is extremely difficult due to the limitations ofimaging tools. In accordance with one embodiment of the presentinventions, the impedance of each of the electrodes 26 is measured andthe impedance measurement is used to determine whether the lead 12 ispositioned correctly. Because different types of tissues have differentconductive properties, by measuring the impedance at each electrode 26a, 26 b, it is possible to determine whether the tissue adjacent to oneof the electrodes 26 a is different from the tissue adjacent to theother electrode 26 b, and thus, whether the lead 12 is correctlypositioned. As discussed in more detail below, a relatively largedifference between the impedances of the electrodes 26 a, 26 b indicatesthat the lead 12 is correctly positioned, as shown in FIGS. 7A, 7D, 7E,and 7F. However, a relatively small difference between the impedances ofthe electrodes 26 a, 26 b indicates that the lead 12 is not correctlypositioned, as shown in FIGS. 7B and 7C.

In the context of SCS, the embodiment shown in FIG. 7A takes advantageof the fact that the cerebrospinal fluid (CSF) 108 between the dura 102and the spinal cord 104 is a good conductor, compared to the tissue inthe epidural space 110, which is not a good conductor. In thisembodiment, the target tissue region is the dorsal column 106 of thespinal cord 104, so the most preferred placement of the neurostimulationlead 12 is in the dorsal region 112 of the epidural space 110 with oneof the electrodes 26 a on the dura 102 of the spinal cord 104. With thelead 12 positioned as shown in FIG. 7A, the impedance of the electrode26 a in contact with the dura 102 is less than the impedance of theelectrode 26 b in the epidural space 110 because of the differentconductive properties of the CSF 108 and the tissue in the epiduralspace 110. As such, the difference between the impedances of theelectrodes 26 a, 26 b will be relatively large, indicating that the lead12 is correctly positioned.

However, a relatively small difference between the impedances of theelectrodes 26 a, 26 b indicates that the lead 12 is not in the correctposition. For example, if the lead 12 is incorrectly positioned and isrelatively medial between the dura and the inner boundary of the spinalcanal (“floating in epidural space”) 110, as shown in FIG. 7B, both ofthe electrodes 26 a, 26 b will have high impedances. In another example,shown in FIG. 7C, the location of the lead 12 may be correct, but theorientation of the lead 12 is incorrect. Since the electrodes 26 a, 26 bpositioned as shown in FIGS. 7B and 7C will have similar impedances, thedifference between the impedances of the electrodes will be small,indicating that the lead 12 is not correctly positioned.

It is advantageous to correctly position the lead 12 on the dura 102during implantation, as shown in FIG. 7A, because after implantation, acapsule of fibrous cells and tissue will grow around the lead 12,thereby lifting the lead 12 off of the dura 102. Implanting the lead 12as close to the dura 102 as possible ensures that, even after the growthof this fibrotic capsule, the electrodes 26 a, 26 b are still closeenough to the dura 102 to be able to effectively apply stimulation tothe dorsal column 106. Another advantage of correctly positioning thelead 12 is that unwanted stimulation of certain tissues near theepidural space 110 can be substantially avoided. Stimulation applied tothe epidural space 110 may affect such tissues as the posteriorligamentum flavum, which is sometimes innervated with pain nerve fibers,resulting in an undesirable pinching sensation in the ligaments. Inaddition, it is undesirable to apply stimulation to the epidural space110 because such stimulation would require high stimulation thresholds,and the stimulation field would not be very well-controlled.

The embodiment shown in FIG. 7D takes advantage of the fact that the CSF108 is more conductive than the posterior longitudinal ligament (PLL)116. In this embodiment, the target tissue region may be the ventralcolumn 114 and/or ventral roots of the spinal cord 104 or the PLL 116,so the neurostimulation lead 12 is implanted within the ventral region118 of the epidural space 110 with one of the electrodes 26 b facing thedura 102 of the spinal cord 104 and the other electrode 26 a facing thePLL 116. Because of the difference in conductive properties between theCSF 108 and the PLL 116, the impedance of the electrode 26 b facing thedura 102 is lower than the impedance of the electrode 26 a facing thePLL 116. As such, the difference between the impedances of theelectrodes 26 a, 26 b will be relatively large, indicating that the lead12 is correctly positioned.

The embodiment shown in FIG. 7E takes advantage of the fact that the CSF108 is more conductive than the spinal cord 104. In this embodiment, thetarget tissue region is the spinal cord 104, and the neurostimulationlead 12 is implanted subdurally between the spinal cord 104 and the dura102 with one electrode 26 a facing the spinal cord 104, and the otherelectrode 26 b disposed within the CSF 108 and facing the inner surfaceof the dura 102. Because of the difference in conductive propertiesbetween the CSF 108 and the spinal cord 104, the electrode 26 a facingthe spinal cord 104 has a higher impedance than the electrode 26 bdisposed within the CSF 108. As such, the difference between theimpedances of the electrodes 26 a, 26 b will be relatively large,indicating that the lead 12 is correctly positioned.

In the context of DBS, the embodiment shown in FIG. 7F takes advantageof the fact that gray matter 122 is a better conductor than white matter120. In this embodiment, the target tissue region may be the whitematter 120 or the gray matter 122, so the neurostimulation lead 12 isimplanted within the brain of the patient with one of the electrodes 26a facing the white matter 120 and the other electrode 26 b facing thegray matter 122. Due to the differences in conductive properties betweenwhite matter 120 and gray matter 122, the electrode 26 a disposed in thewhite matter 120 will have a higher impedance than the electrode 26 bdisposed in the gray matter 122. As such, the difference between theimpedances of the electrodes 26 a, 26 b will be relatively large,indicating that the lead 12 is correctly positioned.

A method 800 for using impedance measurements to determine whether thelead 12 is correctly positioned is discussed with reference to FIG. 8.First, in step 802, the lead 12 is positioned within the patientrelative to the target tissue region. Next, the impedance of each of theelectrodes 26 a, 26 b is measured in step 804. The impedance may becontinuously monitored during implantation. The difference between theimpedances of the two electrodes 26 a, 26 b is then determined in step806. Next, in step 808, the impedance difference is compared to apre-determined threshold value. If the difference between the impedancesexceeds the threshold value, indicating that the lead 12 is in thecorrect position (as discussed above), then the lead 12 is affixed(e.g., by suturing) within the patient in step 810. However, if theimpedance difference does not exceed the threshold value, indicatingthat the lead 12 is not correctly positioned (as discussed above), thenthe processor 81 (see FIG. 4) is configured to provide a suggestion tothe user to re-position the lead 12 in step 812. Re-positioning the lead12 may include linearly displacing the lead 12 to be closer to thetarget tissue region, or may include rotating the lead 12. Afterre-positioning the lead 12, the impedances of the electrodes 26 a, 26 bare again measured (returning to step 804), an impedance difference isdetermined (step 806), the impedance difference is compared to athreshold value (step 808), and (if necessary), the lead 12 is againre-positioned (step 812). Steps 806, 808, and 812 may be repeated untilit is determined in step 808 that the impedance difference exceeds thethreshold value, and thus, that the lead 12 is correctly positioned.

One example of the method 800 will now be discussed with reference toFIGS. 7A-7C. In this example, the target tissue region is the dorsalcolumn 106 of the spinal cord 104. Thus, in step 802, the lead 12 ispositioned within the dorsal epidural space 112. If the lead 12 iscorrectly positioned, as shown in FIG. 7A, then it will be determined instep 808 that the difference between the impedances of the electrodes 26a, 26 b exceeds the threshold value, and the lead 12 will be affixedwithin the patient in step 810. However, if the lead 12 is not correctlypositioned, as shown in FIGS. 7B and 7C, then it will be determined instep 808 that the difference between the impedances does not exceed thethreshold value, and the lead 12 will be re-positioned in step 812. Forexample, the lead 12 shown in FIG. 7B will be linearly displaced to becloser to the dorsal column 106, and the lead 12 shown in FIG. 7C willbe rotated so that one of the electrodes 26 a, 26 b faces the dura 102.

Another example of the method 800 will be discussed with reference toFIG. 7D. In this example, the target tissue region is the ventral column114 of the spinal cord 104, or the PLL 116. Thus, in step 802, the lead12 is positioned with the ventral epidural space 118. If the lead 12 iscorrectly positioned, as shown in FIG. 7D, then it will be determined instep 808 that the difference between the impedances of the electrodes 26a, 26 b exceeds the threshold value, and the lead 12 will be affixedwithin the patient in step 810. However, if the lead 12 is rotated orlinearly displaced relative to the position depicted in FIG. 7D, then itwill be determined in step 808 that the difference between theimpedances does not exceed the threshold value, and the lead 12 will bere-positioned (e.g., linearly displaced or rotated) in step 812.

Another example of the method 800 will be discussed with reference toFIG. 7E. In this example, the target tissue region is the spinal cord104. Thus, in step 802, the lead 12 is positioned subdurally between thedura 102 and the spinal cord 104. If the lead 12 is correctlypositioned, as shown in FIG. 7E, then it will be determined in step 808that the difference between the impedances of the electrodes 26 a, 26 bexceeds the threshold value, and the lead 12 will be affixed within thepatient in step 810. However, if the lead 12 is rotated or linearlydisplaced relative to the position depicted in FIG. 7E, then it will bedetermined in step 808 that the difference between the impedances doesnot exceed the threshold value, and the lead 12 will be re-positioned(e.g., linearly displaced or rotated) in step 812.

Another example of the method 800 will be discussed with reference toFIG. 7F. In this example, the target tissue region may be the whitematter 120 or the gray matter 122. Thus, in step 802, the lead 12 ispositioned within brain tissue. If the lead 12 is correctly positioned,as shown in FIG. 7F, then it will be determined in step 808 that thedifference between the impedances of the electrodes 26 a, 26 b exceedsthe threshold value, and the lead 12 will be affixed within the patientin step 810. However, if the lead 12 is rotated or linearly displacedrelative to the position depicted in FIG. 7F, then it will be determinedin step 808 that the difference between the impedances does not exceedthe threshold value, and the lead 12 will be re-positioned (e.g.,linearly displaced or rotated) in step 812.

A method 900 for using impedance measurements to determine which one ofthe electrodes 26 a, 26 b is closest to the target tissue region isdiscussed with reference to FIG. 9. First, in step 902, the impedance ofeach electrode is measured. Next, in step 904, the electrode closest tothe target tissue region is selected. This selection in step 904 isbased upon the impedance measurement. After the appropriate electrode isselected in step 904, electrical stimulation is conveyed through theselected electrode in step 906. In this manner, the electricalstimulation is directed predominantly at the target tissue region.

For example, referring back to FIG. 7A, the target tissue is the dorsalcolumn 106 of the spinal cord 104. Thus, the electrode 26 a closest tothe dorsal column 106 is selected by measuring the impedances of theelectrodes 26 a, 26 b and choosing the electrode 26 a having the lowestimpedance.

In another example, with reference to FIG. 7D, if the target tissue isthe ventral column 114, then the electrode 26 b closest to the ventralcolumn 114 is selected by measuring the impedances of the electrodes 26a, 26 b and choosing the electrode 26 b having the lowest impedance.Alternatively, if the target tissue is the PLL 116, then the electrode26 a closest to the PLL 116 is selected by measuring the impedances ofthe electrodes 26 a, 26 b and choosing the electrode 26 a having thehighest impedance.

In yet another example, with reference to FIG. 7E, the target tissue isthe spinal cord 104. Thus, the electrode 26 a closest to the spinal cord104 is selected by measuring the impedances of the electrodes 26 a, 26 band choosing the electrode 26 a having the highest impedance.

In still another example, with reference to FIG. 7F, if the targettissue is white matter 120, then the electrode 26 a adjacent to thewhite matter 120 is selected by measuring the impedances of theelectrodes 26 a, 26 b and choosing the electrode 26 a having the highestimpedance. Alternatively, if the target tissue is gray matter 122, thenthe electrode 26 b adjacent to the gray matter 122 is selected bymeasuring the impedances of the electrodes 26 a, 26 b and choosing theelectrode 26 b having the lowest impedance.

In addition to, or instead of, measuring the impedance of each electrode26 a, 26 b, the field potential of each electrode may be measured andused to determine whether the lead is correctly positioned and/or whichof the electrodes is closest to the target tissue region. In general,field potential is a voltage potential measured at an electrode due tocurrent flow within the region surrounding the electrode, the source ofthe current being from other electrodes i.e., the current does not comefrom the two electrodes where the voltage is being measured. Fieldpotentials may be used instead of or in combination with the impedancesand generally the same procedures may be applied as if impedances wereused.

It should be noted that each ring of electrodes carried by the lead 12may include more than two electrodes. By having more than two electrodesin each electrode ring, it may be easier to correctly position the lead12 (e.g., it may be less likely that the lead will need to be rotatedrelative to the target tissue). In addition, more electrodes in eachring may result in greater control of the stimulation field, so that itis less likely that non-target tissue will be stimulated.

It should further be noted that the electrode selected to convey theelectrical stimulation may change over time due, for example, tomigration. Over time, if the the lead 12 has rotated, then the processoris configured to use the impedance measurements to automatically adjustthe current delivery towards the electrode with the closest value to thetargeted tissue resistance. For example, a lead including threeelectrodes in each electrode ring is epidurally positioned to target thespinal cord (e.g., similar to the lead position shown in FIG. 7A), and afirst electrode is initially determined to have the lowest impedance(suggesting it is closest to the dura and CSF). However, it may later bedetermined that a second electrode has a lower impedance. Thus, theprocessor may automatically adjust the current to be redirected to thesecond electrode.

Still further, it should be noted that current steering may be used toconvey the electrical stimulation through two selected electrodes. Forexample, if a lead including three electrodes in each electrode ring isused, it may be determined that a first electrode and a second electrodeboth have lower impedances than a third electrode, but that impedance ofthe first electrode is slightly higher than that of the secondelectrode. In this case, the processor may be configured to employcurrent steering to direct fractional amounts of the total current in amathematical relation to the impedance of the first and secondelectrodes.

Although particular embodiments of the present inventions have beenshown and described, it will be understood that it is not intended tolimit the present inventions to the preferred embodiments, and it willbe obvious to those skilled in the art that various changes andmodifications may be made without departing from the spirit and scope ofthe present inventions. Thus, the present inventions are intended tocover alternatives, modifications, and equivalents, which may beincluded within the spirit and scope of the present inventions asdefined by the claims.

What is claimed is:
 1. A system for use with at least two electrodespositioned within a patient proximate to a target tissue region havingdistinguishable conductive properties, the system comprising: circuitryconfigured to determine at least one value for each of the at least twoelectrodes based on the conductive properties of the tissue; and aprocessor configured to, based on the at least one determined value,identify information related to electrode position with respect to thetarget tissue region for use in providing a therapy to the patient. 2.The system of claim 1, further comprising a segmented ring electrode,wherein the segmented ring electrode includes the at least twoelectrodes.
 3. The system of claim 1, further comprising aneurostimulation lead, wherein the neurostimulation lead includes the atleast two electrodes.
 4. The system of claim 1, wherein the informationrelated to electrode position with respect to the target volume includesinformation regarding at least one of the at least two electrodespositioned closest to the target tissue region.
 5. The system of claim1, wherein the information related to electrode position with respect tothe target volume includes information for positioning at least one ofthe at least two electrodes closer to the target tissue region.
 6. Thesystem of claim 1, wherein the at least one value includes at least oneimpedance value.
 7. The system of claim 1, wherein the at least onevalue includes at least one field potential value.
 8. The system ofclaim 1, wherein the target tissue region comprises at least one of aspinal cord, a posterior longitudinal ligament, a white matter, or agray matter.
 9. A non-transitory machine-readable medium includinginstructions, which when executed by a machine, cause the machine toperform a process using at least two electrodes positioned within thepatient proximate to a target tissue region having distinguishableconductive properties, wherein the instructions, which when executed bythe machine, cause the machine to: determine at least one value for eachof the at least two electrodes based on the conductive properties of thetissue; and based on the at least one determined value, identifyinformation related to electrode position with respect to the targettissue region for use in providing a therapy to the patient.
 10. Thenon-transitory machine-readable medium of claim 9, wherein theinformation related to electrode position with respect to the targetvolume includes information regarding at least one of the at least twoelectrodes positioned closest to the target tissue region.
 11. Thenon-transitory machine-readable medium of claim 9, wherein theinformation related to electrode position with respect to the targetvolume includes information for positioning at least one of the at leasttwo electrodes closer to the target tissue region.
 12. Thenon-transitory machine-readable medium of claim 9, wherein the at leastone value includes at least one impedance value.
 13. The non-transitorymachine-readable medium of claim 9, wherein the at least one valueincludes at least one field potential value.
 14. The non-transitorymachine-readable medium of claim 9, wherein the target tissue regioncomprises at least one of a spinal cord, a posterior longitudinalligament, a white matter, or a gray matter.
 15. The non-transitorymachine-readable medium of claim 9, wherein a segmented ring electrodeincludes the at least two electrodes.
 16. The non-transitorymachine-readable medium of claim 9, wherein a neurostimulation leadincludes the at least two electrodes.
 17. A method performed using atleast two electrodes positioned within the patient proximate to a targettissue region having distinguishable conductive properties, the methodcomprising: determining at least one value for each of the at least twoelectrodes based on the conductive properties of the tissue; and basedon the at least one determined value, identifying information related toelectrode position with respect to the target tissue region for use inproviding a therapy to the patient.
 18. The method of claim 17, whereinthe identifying information related to electrode position with respectto the target volume includes identifying at least one of the at leasttwo electrodes positioned closest to the target tissue region orincludes identifying information for positioning at least one of the atleast two electrodes closer to the target tissue region.
 19. The methodof claim 17, wherein the determining at least one value for each of theat least two electrodes based on the conductive properties of the tissuevolume includes measuring an impedance or includes measuring a fieldpotential.
 20. The method of claim 17, wherein the target tissue regioncomprises at least one of a spinal cord, a posterior longitudinalligament, a white matter, or a gray matter.